1. Field of the Invention
This invention relates generally to an apparatus and method for bending of tubing and more particularly to an apparatus and method for preforming of tubing to a desired curvilinear shape to provide for improved quality of tube bend.
2. Description of the Prior Art
Current rotary draw bending of tubes and pipes (pipe will be deemed included herein in references to tube or tubing) contemplates three sets of tools commonly used to form a tube bend. The tools are (i) a rotary bending form, also called a bend die, (ii) a clamp die, and (iii) a pressure die, sometimes referred to as a follower die. In current rotary draw bending practice the tube to be bent is secured between the bend die and the clamp die and aligned against the pressure die. As the bend die rotates, the clamp die rotates with it, the tube being formed in the arc defined by the bend die. The pressure die prevents the trailing portion of the tube from radial movement. The pressure die may remain fixed or it may move forward with the tube to eliminate sliding contact. The pressure die may also include rollers to eliminate sliding contact between the tube and the pressure die.
The quality of the resultant bent tube is determined by the amount of flattening on the outside of the bend, the amount of wall thinning on the outside of the bend, and the existence and degree of wrinkling or buckling on the inside of the bend.
To improve the quality of the bend, a mandrel may be placed on the inside of the tube to support it from flattening and to help prevent wrinkles from forming. Mandrel support of the tube requires use of lubricants to reduce drag, additional tooling to be set and adjusted, cleaning after bending, machine length limitations, and extra time in loading.
Another practice used to improve bend quality is to use multi-radius or parabolic-shaped grooves in each of the pressure die and bend die to contort the tube into a generally elliptical shape during bending. A generally elliptical-shaped tube is more resistant to flattening and wrinkling of the tube during bending. This method allows bends to be made without a mandrel within certain ranges of relationships of outside diameter, wall thickness, and bend centerline radius. The relationship of tube outside diameter to bend centerline radius (CLR) is called the "D" of bend. A 2" O.D. tube bent on a 4" CLR would be a 2.times.D bend (CLR/O.D.). The smaller the "D" of bend, the greater the wall thickness must be to support the tube during bending to maintain ovality quality requirements. As the wall thickness increases it becomes more difficult to contort the tube into a parabolic shape. As currently practiced, generation of a parabolic shape in the tube requires a relatively high level of pressure die clamping force and may result in deformation of the tube at the entrance to the bend die. An additional die, commonly referred to as a wiper die, may be placed at the inlet to the bend die to overcome the deformation problem. However, the wiper die increases the friction load on the tube and in turn increases susceptibility to buckling or wrinkling.
Rotary draw bending may be accomplished by powered machinery with simple manual controls or operable by computer numerical control (CNC) systems. Such CNC systems include mechanical, electrical and/or hydraulic apparatus for positioning, clamping and rotating the bend die and further include mechanical and hydraulic apparatus for linear propulsion and rotation of the tube.
Machinery and apparatus reflecting the current state of the art of rotary draw bending of tubes is reflected in the following commercial publications:
Tube and Profile Cold Bending Machines (catalog), Schwarze-Wirtz K. G., Cologne, W. Germany. PA0 Computer Controlled Tube Bending and Tube Forming (catalog), Eagle Precision Technologies, Inc., Ontario, Canada. PA0 Miic CNC Pipe Bender (catalog), Chuo Electric Mfg. Co., Ltd. and Tube Tech, Inc., Taylors, S.C. PA0 Teledyne Pines (catalog), Teledyne, Aurora, Ill. PA0 Conrac Bending Equipment for Tube and Pipe (catalog), Conrac Machine Tool Division, Westminster, Calif. PA0 Tools for Bending, Inc. (catalog), Tools for Bending, Inc., Denver, Colo. PA0 CNC Bender (catalog), Chiyoda U.S.A., Goodlettsville, Tenn.
Stange, et al. U.S. Pat. No. 4,765,168 discloses a tube bending apparatus in which a tube is clamped between a u-shaped bend die and a cooperating pressure die. The pressure die advances in a linear direction as the bend die is rotated. The tube groove supplied in the pressure die and bend die have cross-sectional radii of curvature corresponding to that of the tube to be bent. The radius of curvature at the interior of the tube groove at the bend section is less than the outside radius of the tube to be bent. The radii of curvature of the sides of the groove tube have a cross-sectional radius of curvature greater than the outside radius of the tube to be bent with the side surfaces undergoing a smooth, gradual transition into the bottom surface. The bend die and linear pressure die disclosed in Stange allow the tube to assume an elliptical shape during bending.
Hamlin U.S. Pat. No. 3,242,710 teaches the use of a clamp die clamped to the tube to be bent, such clamp die progressing the tube to be bent tangentially in the direction of the bending form. In addition to the clamp die disclosed, Hamlin discloses one or more rollers disposed along the tubular stock exterior of the bend, one roller being advanced during the bending process to a location tangential to the bend die.
Robinson U.S. Pat. No. 1,510,162 describes a pipe bending machine having a pair of rotating bending rollers, each roller having concave grooves which correspond to the general shape of the tube to be bent. The tube to be bent is filled with sand prior to bending in the preferred embodiment in order to prevent collapsing of the tube.
Newhall U.S. Pat. No. 2,996,100 discloses a method for restricting the thinning of the tube metal in the outer wall during bending by applying against the outer face of the outer wall a temporary metal section of greater tensile strength than the metal of the pipe. The temporary metal section is bent with the tube.
Robinson U.S. Pat. No. 1,510,162 discloses a pipe bending machine having one or more rollers containing concave grooves contained between plates with a bending block for bending the tube.
Myer, et al. U.S. Pat. No. 3,456,482 describes a mandrel to be placed in a tube, said mandrel including a relatively rigid support member and a wear-resistant member, the wear-resistant member being constructed of plastics, ceramics, or carbides and having an external surface coated with lubricant.
Rothanburger U.S. Pat. No. 4,355,528 discloses a manually operated device for bending metal tube including a short cylindrical segment, a pivotal lever, and a pressing block mounted on the lever. The cylindrical segment has an exterior circumferential groove which determines the bending radius of the tube to be bent.
Kowal U.S. Pat. No. 4,380,922 discloses a manually operated tube bender comprised of a stationary bend die, a forming member which rotates about the bending axis, a lever attached to the stationary bend die, a lever attached to the forming member, a scale on the stationary bend die, and an indicator on the rotating forming member. Movement of the forming member about the bending axis is affected by manipulation of the levers.
Peppers U.S. Pat. No. 4,424,699 discloses a tube bender for manual operation. The tube bender includes a mandrel having a bending groove, a forming member mounted to the mandrel to swing about a bending axis of the bending groove, movement of the forming member about the bending axis is effected by manipulation of a pair of handles.
The current technology as disclosed in the commercial publications and the referenced patents indicate the desirability of providing an elliptical shape in the tube to be bent to reduce thinning in the outer surface of the tube to be bent and to reduce wrinkling along the inner surface.